So I picked up a used 6505+ the other day which sounds pretty good but there's a constant noise and I'm hoping someone can tell me if it's just tubes or a bigger problem. I'm not sure if it would be considered hiss or hum it's more like a constant shhhhhhhhh sound which happens when the volume is over 1 and the gain set to anything(even 0), happens when guitar is plugged in or not, the amp is plugged into a house outlet and I have tried several. I can hear it on the lead and rhythm channels but disappears when I turn off 'crunch' on the rhythm channel. I also tried hooking up an hd500 using the 4 cable method and when I use amp sims there is no extra noise and everything sounds good. When I use a preset using the 6505's preamp I get the noise. I'm going to bring it to an amp tech before returning it but any advice appreciated.